$6$ Task Six: Synchronous Cipher $(2$ marks$)$

The following stream cipher operates on character A$-$Z one at a time. A character A$-$Z is mapped

to Z$26=0,1,2,...,25$ in the usual way. For a key k Z$26,$ a key stream k$1$k$2$k$3$ is generated by

the following rule:

For a message $=12...$

$,$ where $$ in $26,$ the ciphertext $=12...$

is calculated as

follows:

Your tasks are:

ci $=$ mi $+$ ki $($mod $26)$

$$ Describe the decryption algorithm.

$$ Implement the above algorithm, for both encryption and decryption.

$$ Encrypt the message $$I LOVE WOLLONGONG" with key $=3.$

$$ Decrypt the ciphertext MQJJ with key $=3.$